A percussion core gun is a core sample taking device which cuts a cylindrical divot from the side wall in an open borehole condition. It is used in a borehole where a core gun supporting body is positioned on a cable, and is lowered to a specified depth or elevation, and a bullet shaped core barrel is propelled by explosives radially outwardly into the wall, cutting a divot which is jammed into and received within the cylindrical cavity for sample storage. Thereafter, the entire mechanism is retrieved to the surface. The core gun fires an assembly which will be described hereinafter as the core barrel, and that in turn is plugged at the back end by a plug or bottom assembly, the two generally being pinned together. They are fired as a unit by initiation of an explosive positioned beneath the barrel and bottom assembly and surrounded by an enclosure which will be defined as the firing chamber. The chamber is sealed by means of an external O-ring on the core barrel or bottom thereof.
The equipment making up the present apparatus ideally is used repetitively. However, it is fabricated in multiple components for easy replacement. The core barrel is subject to substantial wear and tear because it is fired with tremendous momentum, impacting the adjacent sidewall and may wear from that. The present apparatus sets forth a construction of core barrel, plug or bottom and explosive chamber that are supported on a common tool body. Of necessity, some, perhaps most of these components must be made of extremely high quality metals subject to subsequent heat treatment. This is necessary to withstand the shock of use in operation. The parts must be assembled into a close fit and hence precise fabrication is required. Moreover, the precise manufacturing requirements, the hard metals typically used, and the shape of the products define a somewhat expensive manufacturing process. The present apparatus sets forth a type of modular chamber and core barrel and bottom assembly that will readily fit in a bore which is not made with great precision and which is retained therein by locking keys. This markedly reduces the cost of manufacture and also provides quick replacement of chambers or core barrels, depending on the location of wear.
There is another requirement imposed on a percussion core gun assembly. This relates primarily to pulling the core containing barrel free from the adjacent side wall. Retrieval is a problem. It should be recalled that retrieval must be accomplished with downhole equipment without observation. To this end, a retrieval cable must be attached to the core barrel for retrieval to the gun assembly. A new and improved arrangement of the cable and anchoring the cable at two ends thereof to the gun while looping the cable through the barrel is set forth. A clamp located on the cable offset from the center thereof is incorporated. This assists in defining the cable into two separate cable segments which are dissimilar in length. The two cable lengths cooperate with end located eyelets for anchoring purposes so that a first cable is pulled. If that cable section breaks, there remains a second cable for pulling. That is located so that the second cable can pull at a different angle, and thereby more readily dislodge the stuck core barrel.
Such a cable arrangement is also useful at the time the core gun is operated to fire the core barrel in a washout zone. Washouts may occur where the diameter of the borehole is quite large, sufficiently large that the core barrel is unable to make operative connection with the sidewall. In that instance, the core barrel will be fired at some substantial velocity, and travel to the end of its tether, and likely break one of the two segments of the cable which anchor the core barrel. When this occurs, the first may break, but the second cable, being somewhat longer, will tend to hold the core barrel and enable its retrieval. The first cable segment may break because it is shorter but it absorbs substantial energy and reduces the velocity so that the second cable segment is permitted to retrieve the core barrel.
The cable connector of the present disclosure sets forth a simple cable which has eyelets at both ends and a sleeve crimped at a central portion but located away from the precise center. This cable connection arrangement thus provides greater reliability in retrieval after firing.
While the foregoing sets out certain problems and describes somewhat the features of the present apparatus, greater understanding thereof will be obtained on review of the drawings found below so that the detailed description of the preferred embodiments can be understood.